The invention relates to a method for detecting a tumble flow in a cylinder chamber of an internal combustion engine, with a characteristic tumble value being determined as a quotient of an angular velocity xcfx89FK of the tumble movement and an angular velocity xcfx89Mot of the internal combustion engine.
The charge movement is a parameter both in spark ignition engines as well as diesel engines which essentially influences the combustion. Principally, one distinguishes between swirl, a charge movement about the cylinder axis, and tumble, a charge movement about an axis perpendicular to the cylinder axis, with both charge movements often occurring simultaneously in mixed form. In order to detect both types of charge movement, integrative methods are now generally used on the testing stand for stationary flows, which methods can supply summary results very quickly. In the case of swirl, the integrative measurement methods supply favorable correlations between the combustion result and the swirl measurement, because the swirl is substantially maintained during the piston stroke. In the case of tumble, however, the correlation between the results on the testing stand for the flow and the combustion result leads to a rather confusing picture. The reason is that the tumble is virtually squashed between the piston and the cylinder head base and dissolves into complex flow structures and, finally, into turbulence.
Various types of integrative measurement methods are known. It is known, for example, to measure the swirl on the testing stand for the flow with the help of a moment measuring device or a rotary wing for a number of valve lifts and to determine therefrom a swirl count by integration over the crank angle. Although other flow structures occur in a real engine than on a testing stand for stationary flows, the results can be correlated favorably with the combustion results. Integrative measuring methods are also known for detecting the tumble charge movement in the cylinder chamber. From the publication SAE 97 16 37 with the title xe2x80x9cCorrelation of the Combustion Characteristics of Spark Ignition Engines with the In-Cylinder Flow Field Characterized Using PIV in a Water Analogy Rigxe2x80x9d, JACKSON, N.S., et al. the so-called xe2x80x9cT-piece method xe2x80x9d is known. The cylinder head is placed on a T-piece, with integrative measuring instruments such as rotary wings or moment measuring instruments being arranged in a part of the transverse pipe. The asymmetrical flow from the inlet port subsequently impinges on the transverse pipe in which a swirl flow is produced as a result of the asymmetry. The angle of the tumble can also be detected by rotating the cylinder head until the maximum measured value is reached.
It is further known from DE 41 33 277 A1 to use a rotating ring in the cylinder chamber to detect the tumble, with the rotational axis of said ring being disposed normal to the central axis of the cylinder chamber. In contrast to the other stationary flow measurements, the cylinder chamber is closed off at the bottom by a dummy piston and, in addition, the air is evacuated on the side in the zone of the rotary wing axis. As a result of the asymmetrical impingement of the air jet on the dummy piston a rotational movement is induced which is detected via the rotational speed of the rotating ring. The tumble angle is obtained by turning the position of the rotating ring or the cylinder head towards the position with the maximum ring speed.
From the article xe2x80x9cRxc3xa4umlicher Drallmesser fxc3xcr Drall- und Tumble-Messungxe2x80x9d (Spatial Swirl Measuring Apparatus for Swirl and Tumble Measurement), TIP-PELMANN, G., MTZ 58 (1997), No. 6, page 327 it is known to use a moment measuring instrument for the integrative measurement of the tumble flow which consists of a spherical flow rectifier. As a result of its radially arranged rectifier bores, the spherical rectifier is not only able to detect a moment of momentum about the z-axis, but also a moment of tilt about an axis in the x-y-plane and can thus respond very elegantly to the asymmetry of the flow field. Since all three spatial moments are detected, swirl, tumble and angle of tumble are obtained from a single measurement. The flow resistance of the rectifier and the thus following reaction on the flow field have a disadvantageous effect on the result of the measurement, however.
It is the object of the present invention to provide a method with which the tumble flow in the cylinder chamber of an internal combustion engine can be detected as precisely as possible.
The method in accordance with the invention provides that the asymmetry of a flow field wLDA in a cylinder chamber is detected with a differential measuring method on the testing stand for the flow for a predetermined number of measuring points i and the characteristic tumble value is determined on the basis of the asymmetry. A reduced flow field wi is preferably determined according to the equation
wi=wLDAxe2x88x92{overscore (w)},
with {overscore (w )}being the central axial flow speed of the flow field wLDA. Particularly favorable results can be achieved when the characteristic tumble value is calculated on the basis of the angular velocity xcfx89i for each measuring point i according to the equation             ω      i        =                  w        i                    r        i              ,
with ri being the distance of the measuring point i from the central axis of the cylinder chamber. An element of surface fi is preferably assigned to each measuring point i and the angular velocity of the tumble movement xcfx89FK is calculated according to the equation       ω    FK    =                    ∑                              ω            i                    ·                      r            i            2                    ·                      f            i                                      ∑                              r            i            2                    ·                      f            i                                .  
In contrast to the known integrative methods, a differential measurement method is used in the method in accordance with the invention in order to detect the flow field in the cylinder chamber in its structure. Any random integral parameters can then be derived from said structure.
The measurement is performed with a stationary testing stand for the flow on the pistonless cylinder chamber which opens into a calming vessel. The method in accordance with the invention makes use of the finding that the size of the tumble occurring in the cylinder chamber of the internal combustion engine is proportional to the magnitude of the asymmetry of the axial flow structure in the pistonless cylinder chamber of the stationary flow testing stand.
It is preferably provided that laser Doppler anemometry is used as the differential measurement method, with the measurement being performed in a measuring plane normal to the axis of the cylinder chamber. The measuring volume of a laser Doppler anemometry system is moved through many points in a measuring plane at a distance from the cylinder head base which corresponds to half the bore diameter of the cylinder chamber. The axial flow field is thus detected. In order to obtain measuring results with high relevance, it is advantageous when at least 200 measuring points i are arranged at the same distance from one another in the measuring plane. It has been noticed that in the case of less than 200 measuring points the characteristic tumble values fluctuate to such a high extent that reproducible measurements can hardly be performed.
It is provided for in a preferred embodiment of the invention that the elements of surface fi which are assigned to each measuring point i are arranged in a hexagonal grid. The hexagonal grid ensures that the elements of surface fi are all the same.